The long-term goal of this project is to understand how cell division is regulated to produce the appropriate type and number of neurons within a specific ganglion of the peripheral nervous system. In particular, the project will address the position and timing of neuronal precursors as they divide to form differentiated neurons. These aspects of neurogenesis will be examined in the cardiac ganglion of the frog Xenopus laevis. By concentrating on this extremely simple ganglion, we hope to obtain information that will guide us in understanding neurogenesis in more complex portions of the nervous system. Preliminary experiments have shown that neurogenesis occurred within cardiac ganglia that were isolated in culture. Organ culture, DNA labeling, and autoradiography will be used to determine if neurogenesis within the ganglion is restricted to this short period of time or if it continues throughout the prolonged period of neuron differentiation. Since cardiac neurons are matched to the size of the heart throughout development, the role of heart tissue in stimulating the division of neuronal precursors will be examined in culture. By applying short pulses of labeled analogs of DNA bases to normal developing animals, the length of the S (DNA synthesis) phase of the neuronal cell cycle will be measured. The length of the S phase, together with data on the length of the total cell cycle, will be used to determine the rate of neuronal cell division. This rate of mitotic activity will then be compared with the rate of neuron differentiation at different stages of development.